Terrestrial analog broadcasting will end in Japan in 2011. It is therefore hoped that by 2011 terrestrial analog broadcasting will have transitioned smoothly to terrestrial digital broadcasting and that terrestrial digital broadcasting coverage to areas that can view terrestrial analog broadcasting will have reached 100%. An IPTV (Internet Protocol Television) service (terrestrial digital IP rebroadcasting service) that uses an NGN (Next Generation Network) network as an IP network has been proposed as a means for meeting these demands. An IPTV service is a form of service for distributing digital TV broadcasting by way of an IP network.
A typical network is of the Best-Effort type that does not guarantee QoS (Quality of Service), and the quality of image data that are distributed to image viewers in an image distribution service therefore cannot necessarily be guaranteed. QoS is guaranteed when an NGN network is used, and image quality equivalent to that of a broadcast TV service that uses electromagnetic waves can therefore be guaranteed.
In a broadcast TV service that uses electromagnetic waves, the area in which the broadcast can be viewed is computed based on the strength of electromagnetic waves that are transmitted from the transmission station and buildings or topography. Tests of image quality are carried out at several observation points within the area in which viewing is possible, and the image quality is monitored by comparing the test results with the results of computing the theoretical image quality.
In contrast, in an IPTV service, there is a plurality of providers from the transmission of content until the reception in the TV receiver of each residence. As a result, image quality must be monitored between each of the providers.
For example, it is established in the ITU-T recommendations that five monitor points for monitoring image quality are to be provided. FIG. 1 is an explanatory view showing the monitor points in the ITU-T recommendation. Monitor points PT1-PT5 are shown in FIG. 1.
As shown in FIG. 1, a communication system for providing an IPTV service includes: content provider 101, service provider (central headend) 102, local content acquisition 103, service provider (regional headend) 104, network provider 105, and end user 106.
Image data are distributed to end user 106 by way of a path composed of content provider 101, service provider 102, and network provider 105 or by way of a path composed of local content acquisition 103, service provider 104, and network provider 105.
Monitor point PT1 is provided between content provider 101 and service provider 102 or between local content acquisition 103 and service provider 104. Monitor point PT2 is provided between service provider 102 and IP core network 105a within network provider 105, or between service provider 104 and IP core network 105a. 
Monitor point PT3 is provided between IP core network 105a and IP access network 105b within network provider 105. Monitor point PT4 is provided between VOD (Video on Demand) server (Edge VOD Server/multicast replication point) 105c on IP access network 105b and the gateway (Home Gateway) 106a of end user 106. Monitor point PT5 is provided between STB (Set-top box) 106b of end user 106 and TV 106c of end user 106.
The following explanation regards the method of monitoring image quality that is implemented at monitor points PT3 and PT4. FIG. 2 is a block diagram showing the configuration of an image quality monitoring apparatus for realizing this monitoring method.
IP packet header analysis unit 11 extracts packets for monitoring image quality from IP packets T1 that are transferred over a network (not shown) and supplies the extracted packets as IP packets T2. For example, when IP packets T1 are data that have been converted to IP multicast packets, IP packet header analysis unit 11 extracts IP packets T1 that have a predetermined multicast address.
UDP/RTP packet header analysis unit 12 separates IP packets T2 into TS packets T3 and FEC (Forward Error Correction) packets T4 based on the destination port numbers of the UDP packet headers in IP packets T2. TS packets T3 are data in which a plurality of TS packets that are image data are converted to RTP, and FEC packets T4 are data in which FEC data for implementing error correction of the image data have been converted to RTP packets.
Of the plurality of TS packets within TS packets T3, FEC packet analysis unit 13 analyzes the headers (TS packet headers) of TS packets for which error correction by FEC packets T4 is possible and supplies the TS packet headers and FEC packets T4 as FEC data T6.
FEC buffer 14 reorders each of TS packets within TS packets T3 in the correct order based on time information within RTP packet headers of TS packets T3. FEC buffer 14 delays TS packets T3 in which the TS packets have been reordered until a timing at which error correction by FEC data T6 is possible and supplies the delayed TS packets T3 as TS packets T5.
Error correction unit 15 carries out error correction of TS packets T5 based on FEC data T6 and supplies TS packets T5 that have undergone error correction as TS packets T7.
TS header analysis unit 16 extracts image packets T8 in which image data have been converted to packets from TS packets T7.
Image quality estimation unit 17 uses the image coding parameters and image packet headers of image packets T8 to estimate the image quality of the image data and supplies image quality estimation information T9 that indicates the image quality.
The technology for realizing this type of monitoring method is an image quality estimation apparatus disclosed in Patent Document 1.